The present invention relates to the deaeration of water abstracted from a stretch of water, for example the sea, or from an underground water-bearing stratum for a use in which the presence of oxygen would lead to serious disadvantages, particularly corrosion. A particularly important use is the supply of water for assisted recovery of hydrocarbons.
It is known that water deaeration can be carried out in vacuum towers or in towers employing entrainment by a gas stream.
The principle of the vacuum tower consists in reducing the pressure of the gas phase down to the vapour pressure of water (0.035 bar to 25.degree. C.) by means of vacuum pumps. The water to be degassed enters at the top of a stage, being distributed over the whole cross-section of the tower by means of a perforated distributor or a system of atomizing nozzles. The contact surface between water and gas along the tower is provided by flow over packings such as rings, saddles, woven media, and the like. Conventional towers generally incorporate one to three stages, each three to four metres high, with vacuum takeoff arranged in the intermediate spaces. From an economic standpoint it is preferable to employ two-stage towers and to carry out a final degassing chemically in a capacity arranged at the bottom of the tower.
In towers employing entrainment by a gas stream, entrainment of the dissolved gases present in the liquid phase is produced by subjecting the latter to a countercurrent passage of another gas whose content of the gas to be removed (in this case oxygen) is as low as possible: natural or inert gas. These towers are quite similar to the vacuum towers. The water to be degassed enters at the top of the tower and the entrainment gas just above the storage capacity arranged at the bottom of the tower, the ratio of the volumes of entrainment gas and water to be treated being between 1 and 2. As in the case of vacuum towers, the contact surface between the water and gas can be provided by packings, but it can also be produced by a series of plates. In order for the number of plates to be economically acceptable, a final degassing is generally carried out by a chemical method, as in the case of vacuum towers.
It has recently been proposed to carry out the pumping and the degassing of the water simultaneously by means of a process known as "gas-lift", consisting in injecting an inert gas under pressure at the bottom of an upright column immersed in the stretch of water and open at its bottom end, so that this inert gas drags the water present in this column upwards and at the same time releases most of the oxygen dissolved in the water which it entrains.
This process, theoretically attractive, requires very bulky plants which are not readily compatible with the very restricted space available on a platform at sea.
In point of fact, the two functions which the injected gas is to fulfil, that of lifting the water by pumping and of deaerating it, are contradictory; when the intention is to deaerate the water, the volume of the injected gas does not need to be great, but because of the fact that the energetic efficiency of this gas for lifting the water is low, large quantities of injection gas need to be conveyed; the gas compression and separation units are then very bulky.